It is important in the commercial polymerization of vinyl monomers to perform the reaction in as short a period of time as possible and yet to obtain polymer product of high quality and in good yield. The process of the present invention represents an advance over the art known by accomplishing such desirata with conventional equipment, using increasingly higher, multistage temperature stages during polymerization and a mixture of initiators including a defined initiator particularly effective during the earliest, low temperature stage, and a different initiator more effective at the higher temperature. The process described is applicable to any of the bulk, suspension, or emulsion techniques.